1. Technical Field
The invention relates to gas and oil well swabs used in raising liquids trapped within a well casing. More particularly, the invention relates to such a swab having an external operating valve which opens and closes the fluid flow passage through the swab for its operation and which has a pressure relief valve to prevent damage to the flexible seals.
2. Background Information
Various oil and gas well swabs have been developed which are slidable within a gas well casing or fluid delivery tube for raising the oil from the bottom of the well by using the pressure generated in the bottom of the well casing and/or delivery tube. These devices are commonly referred to as "lifters" or "rabbits" and usually will include a rigid body having external seals mounted thereon, usually formed of a flexible or friction reducing material, to provide a sliding sealing relationship with the internal wall of the casing and/or delivery tube.
These prior art swab devices incorporate a plunger element or control valve and associated rod which is slidably mounted within an internal fluid passage within the swab. A valve mechanism which also is mounted within the body of the swab is actuated to close the internal flow passage when the swab has reached a predetermined depth in the accumulated liquids where the hydrostatic pressure is sufficient to move the valve to its closed position. When the valve is closed, the natural gas existing in the well accumulates below and acts upon the swab as a piston to move the swab and all liquids located above the swab to the well surface. Some examples of these swabs are shown in U.S. Pat. Nos. 4,070,134, 4,813,485 and 4,923,372.
However, problems can occur with such prior art swabs, and in particular with the actuating mechanism therefore, due to the relatively harsh environment in which the swabs are operated, in that the well casings usually will contain sand, dirt and similar debris which can easily effect the operation of the swab and in particular the control rod and valve thereof, especially when the actuating rod lies within a complementary shaped bore within the swab or within retainers, which are subject to hangup due to the accumulation of sand and other particles between the sliding surfaces of the rod and/or bore, interior guide members or the like.
Many of these problems are overcome by the swab of the present invention which use a valve which is located externally at the bottom end of the swab, which valve is closed upon contact with a stop member at the bottom of the well, and does not rely upon the internal fluid pressure within the bottom of the well casing for closing the valve. Furthermore, the hangup problem is eliminated by using a specially shaped actuating rod which provides sliding line contact only with the interior surface of the swab bore, and provides sufficient cross-sectional flow areas around the actuating rod within the well bore for the unrestricted free flow of fluid therethrough. Numerous prior art swabs use external rod-like members for actuating various valves and control mechanisms within the swab such as shown in U.S. Pat. Nos. 2,642,002; Re 23,698; 2,674,951; 2,821,142; 2,878,754; 2,937,598; 3,055,306; 3,273,504; 3,329,211; 3,424,093; 4,363,606; 4,531,891 and 4,984,969. However, none of these external rods or valves are used for opening and closing the open end of a flow through passage or bore within the valve as in the present invention.
U.S. Pat. No. 3,090,315 shows a piston having a control rod which appears to reduce the sliding friction with the internal surface of the bore forming wall by the use of radially extending pins or arms on the rod.
Another problem with existing swabs is that many of the swabs use flexible bell-shaped elastomeric seals such as shown in U.S. Pat. No. 4,528,896, to provide a generally fluid tight seal between the swab and inside surface of the well casing to permit the fluid pressure beneath the swab to raise the swab within the casing. However, occasionally a swab will bindup within the well casing, such as when the flexible seals are at a junction between casing sections or experiences irregularities on the internal surface of the casing. This will result in the pressure beneath the swab continuing to increase until it reaches a sufficiently great pressure to blow by the flexible seal, either by turning the seal upwardly on itself or blowing a hole through the seal, in order to escape to the area above the swab and equalize the pressure within the well casing. This then may require that the swab be retrieved or "fished out" from the bottom of the well, all of which results in increased well operating costs and reduced efficiency.
Accordingly, the need exists for an improved gas and oil well swab which provides a simple, effective and efficient operating valve for opening and closing the main fluid flow passage through the body of the swab, which reduces hangup of the operating control rod within the well body, and which prevents excess pressure from damaging the flexible swab seals within the well casing.